In connection with load-carrying vehicles powered by a diesel engine, there exists a general need for reducing, as much as possible, the emission of harmful pollutants from the engine exhaust gases. These emissions are primarily nitrogen oxide compounds (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC). In order to reduce these emissions, various measures can be taken. For example, it is known that the design of the combustion chamber in the engine cylinder and the process of injecting the engine fuel can be adjusted so as to minimize these emissions. In those cases where the engine is equipped with a turbocharger, the emission of NO.sub.x compounds may be reduced by cooling the air fed into the engine (known as "intercooling").
In connection with gasoline driven engines, the exhaust gases are normally purified by means of a catalytic converter located in the exhaust system. Because a diesel engine is operated with an air surplus, however, the three-way catalyst cannot be used for the reduction of NO.sub.x compounds from a diesel engine.
Due to increasing environmental requirements and the expected demands of future legislation, the need for reducing the emissions, primarily of NO.sub.x compounds, from diesel engines has grown ever stronger. In this context, it is known that the amount of NO.sub.x compounds from a diesel engine can be reduced by equipping it with something known as an EGR system (Exhaust Gas Recirculation), by means of which a certain amount of exhaust gases can be returned from the engine exhaust to its intake. The amount of NO.sub.x compounds generated in a diesel engine is principally exponentially proportional to the temperature inside the combustion chamber, and, by using an EGR system, the local temperature during combustion can be lowered by dilution with exhaust gases (CO.sub.2 and H.sub.2 O). This in turn leads to less creation of NO.sub.x.
A diesel engine may be provided with an EGR system by connecting a separate line between the ordinary exhaust outlet of the engine and a point close to the fresh air intake of the engine. In this line, a controllable valve can be arranged, which is also connected to a control unit. In dependence of the existing engine operating conditions, particularly its rotational speed and load, the control unit will determine the degree of opening of the valve, i.e. the amount of EGR gases to be recirculated to the engine air intake. A certain amount of EGR gases will then be fed from the engine exhaust side to its intake side, by means of the fact that the exhaust side pressure is normally higher than the intake side pressure, thus creating a natural "propulsion pressure" for the EGR gases.
In those cases where a diesel engine having an EGR system is utilized together with a turbocharger unit, a problem will thus be created by the fact that for most of the operating points, a higher pressure exists after the turbocharger compressor (i.e. at the point of the engine intake manifold where the incoming fresh air is fed to the engine) than at the engine exhaust outlet. This, in turn, means that a recirculation of EGR gases will not be possible, as there will not be any natural propulsion pressure from the engine exhaust to its intake side. In this manner, no EGR gas flow can be injected into the engine.
In accordance with the prior art, this problem can be solved by providing the turbocharger with variable turbine geometry. One arrangement using this solution is shown in Japanese Patent Application No. 08270454 A, disclosing a diesel engine having a turbocharger comprising adjustable guide vanes, which, depending on the engine operating conditions, can be adjusted to a certain position by means of a control unit. In this way, a sufficiently high pressure can be created on the engine exhaust side whereby a required amount of EGR gases can be recirculated to the intake side.
A substantial disadvantage of this known apparatus relates to the fact that it will entail an impaired gas exchange in the engine. Consequently, there is a need for engine arrangements providing a flow of EGR gases without impaired gas exchange in a diesel engine equipped with a turbocharger.
An object of the present invention is to provide an improved arrangement for an internal combustion engine, particularly a diesel engine equipped with an EGR system and a turbocharger, and providing an adequate propulsion pressure for the EGR gases, so as to achieve a reduction of the NO.sub.x emissions of the engine.